Chapter 5
Transport layer
With special emphasis on Transmission Control
Protocol (TCP)
Contents
• Need for TCP
• TCP functions
– Segmentation
– Reliability
– Flow-control
– Multiplexing
– Connection establishment
• TCP Header
• UDP
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
The need for a Transport layer
• The network layer (IP) sends packets of data to
their correct destinations with best effort
– Packets may get lost in transit
• The maximum packet size in IP is 65,536 bytes
– What happens if the application wants to send a file
of size 5,000,000 bytes?
• Port addressing
– Distinguish between applications on computers
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
The Transport layer in the stack
http
smtp
sip
Other
application
protocols
Application
layer
Transport layer
Transport
layer
IP
Network
layer
Local network
Data Link
layer
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Transport Control Protocol (TCP)
• These (and other related jobs) are performed
by the Transport layer
• TCP is a highly reliable host-to-host transport
layer protocol over packet switched networks
– Defined in RFC 793 (Sep 1981)
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Transport layer - reference
• TCP
– RFC 793 (Sep 1981)
• UDP
– RFC 768 (Aug 1980)
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP – Overview of operation
• At the sending end, the application creates
the data to be sent and passes it to its TCP
module for transmission
• At the receiving end, TCP collects all fragments
from IP, reassembles them into the original
data and passes it to the receiving application
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions
• TCP provides a reliable application-toapplication communication service over a
network of arbitrary complexity and
unreliability
– Applications (such as browsers and email) simply
pass the data to be transferred to TCP and leave
all communication complexities to TCP
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions
• TCP corrects for all possible imperfections of
IP
• Therefore TCP has to perform many tasks
– Segmentation
– Reliability
– Multiplexing
– Flow-control
– Connection establishment
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions - segmentation
• TCP allows IP to transfer arbitrarily large
data blocks
– Accomplished by breaking data into
segments
– A sequence number is assigned to each
datagram
– Sequence numbers help receiver order
datagrams even when received out of order
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Sequence numbers
Application layer data
Data length = 930 Bytes
To transport layer
Segmentation by transport layer
Sequence number = 1
Length = 300
Sequence number = 301
Length = 320
Sequence number = 621
Length = 310
Sequence number of a TCP
segment
=
To IP for
transmission
TCP segment 3
To IP for
transmission
TCP segment 2
To IP for
transmission
TCP segment 1
sequence number of the previous segment
+ length of previous segment
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions - segmentation
• Advantages
– Errors are less likely in smaller segments
– Less retransmission if error is introduced in a
segment
– Easier for routers to hold segments in memory if
onward route is busy
• Disadvantages
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions - reliability
• TCP recovers from network damage to data
• Basic mechanism
• Datagrams may also get duplicated
– Sequence numbers help identify these duplicates
13
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Sequence numbers and reliability
Sender
Receiver
TCP segment 1
TCP segment 1
Sequence number = 1
Length = 300
Sequence number = 1
Length = 300
ACK 301
TCP segment 2
TCP segment 2
Sequence number = 301
Length = 320
ACK 622
Sequence number = 301
Length = 320
Sender times out
TCP segment 2
TCP segment 2
Sequence number = 301
Length = 320
ACK 622
Sequence number = 301
Length = 320
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions - reliability
• Datagrams may get seriously damaged during
transmission
– If datagrams are further fragmented by IP, some IP
fragments may get lost during transmission
– Such damage is corrected by adding a checksum
to each datagram and comparing it at the receiver
15
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions - multiplexing
• Modern computers are capable of multitasking
• TCP supports multiplexing by providing
multiple port addresses within each host
• A network address and port address together
is called a socket
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Port addresses and multiplexing
Windows file share
131.247.95.252
Port 445 (smb)
Port
56244
Port
52279
Quicken application server
12.149.160.223
Port 443 (https)
Port
52336
Port
52277
Yahoo web server
68.142.213.132
Port 80 (http)
Client PC
Port
131.247.95.132 61331
USF file server
131.247.100.8
Port 21 (ftp)
College of Business DNS server
131.247.92.208
Port 53 (dns)
17
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP ports and airport gates
ru n
wa
y
Gate B25 to
Lansing, MI
Airport code
GSO
Gate B26 to
Phoenix, AZ
Data packets
Port 1856
Web to yahoo
cable
Computer address:
216.149.56.68
Port 1857
IM to MSN
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP port assignment
• On the sender side, the operating system
assigns one of the free ports to an application
that requires network connectivity
• How do you know what port to connect to on
the receiver side?
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Standard ports
• Standard ports
– Assigned by IANA: Internet Assigned Numbers
Authority
• To see list
– In Windows:
• C:/Windows/System32/drivers/etc/services
– In UNIX/ Linux
• /etc/services
• Used ports displayed by netstat utility
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Standard ports
• Typically ports 1 – 1023 are reserved for
defined services
– Applications may use the remaining port numbers
1024 – 65535
• Common ports
– 80 : web (http)
– 25 : email (smtp)
– 443: SSL (https)
– 445: microsoft-ds (smb)
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
etc\services file
22
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Viewing ports usage with netstat
Ports on local desktop
Ports on remote servers
23
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Viewing ports usage with netstat
Invoking application
Invoked port
24
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions – flow control
• If the receiver is slow, it will not be able to
process the packets reaching it
• Therefore, TCP enables the receiver to limit
the sender’s data rate
• If sender is slow, it only tests the patience of
receivers
25
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
V. simple flow control mechanism
Sender
Receiver
TCP segment 1
TCP segment 1
Sequence number = 0
Length = 300
ACK 301
Sequence number = 0
Length = 300
TCP segment 2
TCP segment 2
Sequence number = 301
Length = 320
ACK 621
Sequence number = 301
Length = 320
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions – flow control
• Flow control mechanism shown earlier is
called stop-and-wait
– Sender waits for ACK before sending next
datagram
• TCP flow control is accomplished by returning
a “window” with every ACK
• “Window” indicates how many bytes of data
the sender may transmit before receiving any
more ACK
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP flow control with window size
Sender
Receiver
ize
Window S
700
TCP segment 1
TCP segment 1
Sequence number = 1
Length = 300
Sequence number = 1
Length = 300
TCP segment 2
TCP segment 2
Sequence number = 301
Length = 320
ACK 621
Sequence number = 301
Length = 320
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Sliding window
• Flow control is the regulation by the receiver
of the amount of data the sender may send
• Creates a “sliding window” at the sender
• Packets that have received permission for
transmission are within the window
• Window slides as receiver acknowledges
packets or modifies window size
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Sliding window
Sender
Receiver
Window Size 700
Seg 1
SN = 1
Len = 300
Seg 2
SN = 301
Len = 300
Seg 3
SN = 601
Len = 300
Segments permitted
for transmission
“SENT window”
Seg 4
SN = 901
Len = 300
Segments waiting
for permission
Seg 1
SN = 1
Len = 300
Seg 2
SN = 301
Len = 300
Seg 6
SN = 1501 More data
Len = 300
Segments
waiting
for
permission
Seg 3
Seg 4
SN = 601
SN = 901
Len = 300 Len = 300
Seg 5
SN = 1201
Len = 300
ACK 601; Window Size 900
Seg 13
SN = 1
Len = 300
Seg 2
SN = 301
Len = 300
Segments successfully
transmitted
Seg 3
SN = 601
Len = 300
Seg 4
SN = 901
Len = 300
More data
Seg 5
SN = 1201
Len = 300
Segments permitted
for transmission
“SENT window”
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP functions – connection
establishment
• Sequence numbers are core part of TCP
• It is not a good idea to reuse the same
sequence numbers in succession
• Hence, before communication starts, sender
and receiver negotiate a set of sequence
numbers to use in TCP
– Called 3-way handshake
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
Using an initial sequence number
Sender
Receiver
TCP segment 1
TCP segment 1
Seq no = 1620789
Length = 300
ACK 162
789 + 300
1090 (1620
+ 1)
Seq no = 1620789
Length = 300
TCP segment 2
TCP segment 2
Seq no = 1621090
Length = 320
ACK
621090 +
1621411 (1
320 + 1 )
Seq no = 1621090
Length = 320
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
3-way handshake
Sender
Receiver
SYN Flag = 1, ACK Flag = 0
Sequence number = 83441
Acknowledgment number = 0
Relative sequence number = 0
Length = 0
1
SYN Flag = 1, ACK Flag = 0
Sequence number = 83441
Acknowledgment number = 0
Relative sequence number = 0
Length = 0; SYN = 1; ACK = 0
SYN Flag = 1, ACK Flag = 1
SYN Flag = 1, ACK Flag = 1
Sender knows that
receiver has copied the
correct starting sequence
number
2
Sequence number = 2713867
Acknowledgment number = 83442
Relative sequence number = 0
Length = 0; SYN = 1; ACK = 1
Sequence number = 2713867
Acknowledgment number = 83442
Relative sequence number = 0
Length = 0
SYN Flag = 0, ACK Flag = 1
Sequence number = 83442
Acknowledgment number = 2713868
Relative sequence number = 1
Length = 0
3
SYN Flag = 0, ACK Flag = 1
Sequence number = 83442
Acknowledgment number = 2713868
Relative sequence number = 1
Length = 0
Receiver knows that
sender has copied the
correct starting
acknowledgment number
33
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP Header
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Source port
20
21
22
23
24
25
26
27
28
29
Destination port
Sequence number
Acknowledgment number
Data offset
U
R
G
Reserved
A
C
K
P
S
H
Checksum
R
S
T
S
Y
N
F
I
N
Window
Urgent pointer
Options
Padding
Note: Each tick mark represents a bit position
34
30
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP header fields
• Port addresses have 16 bits
– 216 possible ports
– 216 = 65,536 ports possible per host
– If computing resources are available, a single
computer running TCP can support 65,536
simultaneous network connections
• Compare to telephone – no concept of port
– only one network connection possible at a time
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP header fields
• Sequence and acknowledgment numbers have
32 bits
– Sequence number is the sequence number of the
first data byte in the datagram
– Acknowledgments are cumulative
• ACK 1079 implies that all data till byte number 1078
have been received correctly
• Initial sequence number for connection is
chosen from an ascending clock
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Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP header fields
• Data offset is the number of 32-bit words in
the TCP header
• Control fields
– ACK: 1 implies that the value of the
acknowledgment field is meaningful
– RST: Reset the connection
– SYN: 1 implies that the segment is trying to
synchronize sequence numbers
– FIN: Sender has no more data to send
37
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
TCP header fields
• Window size
– Number of data octets the sender of this
information is willing to accept, beginning with
the acknowledged segment
• Checksum
– Verification of TCP header, IP header and TCP data
38
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
User Datagram Protocol (UDP)
• Defined in RFC 768 (1980)
• Many applications do not need TCP, e.g..
– When applications send very small amounts of
data (DNS),
– Prefer speed to reliability (voice)
• In these cases, if we can avoid TCP, we
eliminate the overhead of keeping track of
sequence numbers, window sizes etc.
39
Overview Segmentation Reliability
Flow
Connection
Multiplexing
Header UDP
control
establishment
UDP Header
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Source port
Destination port
Length
Checksum
27
28
29
Note: Each tick mark represents a bit position
40
30
31
Summary
•
•
•
•
•
•
Why segmentation
Why sequence numbers
Why sliding window
Why port numbers
Why three-way handshake
Why UDP
Case study – the financial industry
• Most financial transactions occur on computer
networks
– Securities markets, credit cards, bank transfers
• In securities markets, low latency is
considered very important
– The delays from TCP handshaking and flow control
are unacceptable
• Alternate transport protocols are used
Hands-on exercise
• netstat
• In Windows
– netstat /? Displays help
– netstat –n
– netstat –f
– netstat –b
• Requires administrative privileges
Network design exercise
• Data rate from database transactions
– Conversion from bytes to bits
• Data rate from phone calls